In general, an image sensor is a semiconductor device for converting optical images into electrical signals. The image sensor is mainly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide silicon (CMOS) image sensor (CIS).
A CIS includes a photodiode and a MOS transistor in a unit pixel. The CIS sequentially detects electric signals of each unit pixel in a switching manner to realize an image.
The photodiode region converts a light signal into an electrical signal and the transistor processes the electrical signal. Generally, in a CIS, the photodiode and the transistor are horizontally disposed on a semiconductor substrate.
According to a horizontal type CMOS image sensor, the photodiode and the transistor are horizontally adjacent to each other on the semiconductor substrate. Therefore, an additional region within each pixel area is required for forming the photodiode.